Camera Prespective under Animation screen layout showing mirror Image
I think you may have accidentally scaled your camera to a negative number. With the camera selected, open up the properties panel with N and see if the camera's scale is anything smaller then 0.

If you have scaled the camera, then just make the X axis a positive number again, and your render will work as excepted.  

There really is no reason to have the camera scaled to anything but 1, except if you are looking to flip the image. However if a three axis have a negative number, after changing the camera's scale you will have to reposition the camera.